Aim: To verify the clinical diagnostic criteria for metabolic syndrome (MetS-Criteria) for Japanese (Japanese-Criteria) from the viewpoint of reference intervals (RIs) of metabolic factors. Methods: In healthy subjects selected from 1,379 males and 1,528 females aged 20-65 years, generation-and gender-specific RIs of metabolic factors were determined. Then, from the upper or lower limits of the RIs, new MetS-Criteria were tentatively set (New-Criteria). Results: RIs of some metabolic factors were associated with gender- and/ or generation-specific differences. In both genders, the New-Criteria for subjects in their 40s and over (older adults) were comparable to the Japanese-Criteria, whereas the New-Criteria for these in their 30s and less (younger adults) were not. Levels of cut-off values in the New-Criteria for younger adults were mostly between those of the Japanese-Criteria and recently established MetS-Criteria for adolescents. The prevalence of metabolic syndrome estimated by the Japanese-Criteria and the New-Criteria well corresponded in older adults but significantly differed in younger adults. Conclusion: MetS-Criteria prepared on gender- and generation-specific bases may be necessary. The Japanese-Criteria, which may still have room for minor changes, seem to be valid at least for older adults
however, new MetS-Criteria should be established for younger adults.